Melt chocolate in a large metal bowl over a pan of simmering water. Stir occasionally until smooth. Remove from heat, and stir in the corn syrup. The mixture will become sticky, but keep stirring until very well blended.
Spoon onto a piece of plastic wrap, and wrap tightly. Let stand at room temperature until firm before using. Overnight is best. Use unsweetened cocoa powder on work surfaces to prevent sticking.
